1 © 2007 The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California • www.understandingscience.org Rutherford photo from the Library of Congress A science prototype: Rutherford and the atom In the early 1900s, Ernest Rutherford (Fig. 1) studied (among other things) the organization of the atom— the fundamental particle of the natural world. Though atoms cannot be seen with the naked eye, they can be studied with the tools of science since they are part of the natural world. Investigating atoms To study such small en- tities, Ernest Rutherford relied on alpha particles, which are helium atoms stripped of their elec- trons. Rutherford had found that when a beam of these tiny, positively- charged alpha particles is fired through gold foil, the particles don’t stay on their beeline course, but are deflected (or “scattered”) at different angles (Fig. 2). Rutherford wanted to figure out what this might tell him about the layout of the atoms in the gold foil. Before 1910, Ernest Rutherford and many other scientists had the idea that the positive charge and the mass of an atom were evenly distributed throughout the whole atom, with electrons scattered throughout. You can imagine this model of the atom as a loosely packed snowball (the positive mass of the atom) with a few tiny grains of sand (the electrons) scattered throughout. The idea that at¬oms are arranged in this way can be tested by firing an alpha particle beam through a piece of gold foil. If the idea were correct, then the positive mass in the gold foil would be relatively diffuse (the loosely packed snow) and would allow the alpha particles to pass through the foil with only minor scattering (Fig. 3). Ernest Rutherford’s lab tested the idea that an atom’s positive mass is spread out diffusely by firing an alpha particle beam through a piece of gold foil, but the evidence resulting from that experiment was a complete surprise: most of the alpha particles passed through the gold foil without changing direction much as expected, but some of the alpha particles came bouncing back in the opposite direction, as though they had struck something dense and solid in the gold foil (Fig. 4). If the gold atoms were really like loosely packed snowballs, all of the alpha particles should have passed through the foil, but they did not! Fig. 1. Ernest Rutherford Fig. 2. Fig. 3. Fig. 4.
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